SOIL PH AND IMPLICATIONS FOR MANAGEMENT: AN INTRODUCTION Soil Acidification Series FS170E FS170E | Page 1 | ext.wsu.edu SOIL PH AND IMPLICATIONS FOR MANAGEMENT: AN INTRODUCTION By Carol R. McFarland, Crop and Soil Sciences, Washington State University, David R. Huggins, USDA-ARS, Richard T. Koenig, Associate Dean and Director of Extension, Washington State University Abstract Decreasing soil pH, also called soil acidification, is a growing concern in eastern Washington and northern Idaho. Researchers and farmers have measured soil pH values below 5.0 throughout the Palouse region. Decreasing soil pH has serious implications for the cropping systems of the Palouse. This introduction is the first in a series of fact sheets on soil acidification and introduces the fundamentals of soil pH and acidification. Other fact sheets in the series will cover more specific information on topics such as the influence of pH on pathogens and microbes, herbicide activity, crop nutrition, liming, and variety selection. Table of Contents What is pH? 3 pH in the Soil 3 Soil Acidification 4 Acidification in the Palouse 5 Low pH and Crops 5 Managing Low pH 5 References 6 FS170E | Page 2 | ext.wsu.edu WSU EXTENSION | SOIL PH AND IMPLICATIONS FOR MANAGEMENT: AN INTRODUCTION Soil pH and Implications for Management: An Introduction For many nutrients, optimum availability occurs near neutral What is pH? pH (7.0) and availability decreases as pH values become more extreme. This can lead to nutrient deficiencies in crop plants. Acidic, neutral and alkaline/basic are terms commonly used to Figure 3 shows yellowing between the veins (interveinal describe pH. A measure of the concentration of hydrogen ions chlorosis) of a corn plant due to iron deficiency in a field (H+) in a system, pH is described using a negative-logarithmic where the soil pH is too high. scale. This means that for each whole number step down on the pH scale there are ten times more hydrogen ions, or “acidity,” than the previous step. Neutral pH is 7, while the neutral pH range is generally thought to be between 6 and 8. Alkaline or high pH values are above 8, while acidic or low pH conditions are typically considered to be below 6. Figure 1 shows pH values for common substances from battery acid to lye. Figure 2: General pH effects on nutrient availability and soil biology. Wider parts of the bar indicate the element is relatively more available to plants, while narrow parts indicate decreased availability of the element at that pH. Deviation from neutral pH decreases availability of many essential plant nutrients as well as bacteria populations. (Adapted from Truog 1947). Figure 1. The pH of common items. Each pH unit is a ten-fold increase or decrease of acidity (H+). pH in the Soil In soil, pH is known as a master variable because it influences almost every process in the soil system. The health of crops and other soil life, the availability of nutrients, and the activity of pesticides are all affected by pH . Generally, soil pH below 5 is considered to be very low and extremely acidic for many agricultural crops. A pH too far from neutral, either above (alkaline) or below Figure 3: Yellowing between veins in corn resulting from iron deficiency in high pH soil (from Koenig- used with permission). (acidic), will make essential nutrients less available to plants. Figure 2 shows how pH affects nutrient availability in soil. FS170E | Page 3 | ext.wsu.edu WSU EXTENSION | SOIL PH AND IMPLICATIONS FOR MANAGEMENT: AN INTRODUCTION In addition to decreasing nutrient availability, low pH can The material and history of a soil’s formation affects the degrade clay minerals in the soil that contain aluminum. This likelihood of acidification. Climate is also a main driver of process releases aluminum (Al), which is toxic to plants and natural acidification; atmospheric CO2 slightly acidifies can interfere with root growth and crop development. rainwater, and, over geologic time, contributes to soil Absorption of aluminum and other toxic elements under low acidification. Acidic conditions are also created by removal of pH conditions results in yellowing, stunting, and general base cations such as calcium (Ca2+) and magnesium (Mg2+) reduced vigor, especially in early growth stages. This is shown from the root zone, either by leaching through the soil profile in Figure 4 by the sparse growth of winter wheat at the early or removal with harvested plant material. stage of stem elongation under low pH conditions. As aluminum is released, it will preferentially bind to the cation Typical acidic soils, such as those found in the southeastern exchange sites on soil organic matter and clays. Desirable United States, developed over thousands of years under humid plant nutrients, such as calcium and magnesium, are then conditions and have been naturally acidified. These soils released and become more susceptible to leaching out of the typically have lost most of their ability to hold nutrients. They root zone. This also reduces the over-all nutrient holding have a unique texture and fewer base cations like calcium and capacity of the soil. magnesium in the root zone because the cations have been flushed deep into the soil through the process of leaching. The soils of the Palouse are relatively younger, wind-deposited silt loams in a drier climate and not typical of acidic soils. The Palouse previously had native forest or grass prairie cover and, as recently as the 1950s, its soil pH was near neutral. Differences in native vegetation resulted in local soil properties developing differently. Prairie soils developed in the drier part of the Palouse and tend to be higher in organic matter and concentrations of calcium and magnesium, which help to stabilize soil pH. Alternatively, Palouse soils that were historically forested have lower calcium and magnesium contents, lower organic matter and higher aluminum levels. Figure 4: Reduced vigor, yellowing and patchiness in wheat resulting from low pH and Al toxicity. This leads to a reduced ability to buffer pH changes, thus, such soils can become acidic more quickly. Figure 5 is a general The effects of low soil pH extend throughout the crop map of the Inland Pacific Northwest depicting the zones that production system. Reduced availability of certain nutrients were historically forested. resulting from low pH affects the growth and development of crops and weeds. Pesticides, including herbicides may be less effective and/or persist longer in low pH soil. Low pH can negatively impact the soil microbial community, especially rhizobia symbionts of legumes, other important contributors to nitrogen cycling, and even earthworms. Low pH can alter the soil environment to favor common fungal pathogens and certain weed species. The far-reaching effects of soil pH on the cropping system underscore its importance as a factor influencing crop production and soil quality. Soil Acidification Thirty percent of the earth’s soils are acidic. Most of these soils have been naturally acidified over geologic time. Several Figure 5: The shaded area of the map shows areas of the Inland Pacific factors contribute to natural acidification. Northwest that were historically forested. These soils may become acidic more easily (Koenig et al., 2011, used with permission). FS170E | Page 4 | ext.wsu.edu WSU EXTENSION | SOIL PH AND IMPLICATIONS FOR MANAGEMENT: AN INTRODUCTION Figure 6: When ammonium-based fertilizers are applied to the soil, they quickly convert to nitrate. This nitrification process releases excess hydrogen ions (H+) that make soil more acidic. + Use of nitrogen-based (ammonium/NH4 ) fertilizers has Tolerance to acid soil varies among plants and can be accelerated the rate of soil acidification on agricultural lands. improved through crop breeding. Plants have many adaptations Ammonium in anhydrous ammonia and other nitrogen that can mitigate the impact of extreme pH and accompanying fertilizers is converted to nitrate by soil microbes (Figure 6). nutrient deficiencies or toxicities. This process produces acidity (H+) in the soil. An acidic zone typically develops in reduced tillage systems where fertilizer is Developing aluminum-tolerant wheat varieties that grow better banded at a depth of 3 to 5 inches. This effect can be reduced in low pH conditions is one current objective of the wheat through tillage in the short term, as the excess H+ is breeding programs at WSU. These varieties can help reduce mechanically mixed throughout the plow layer. yield loss associated with acidic conditions. However, even using aluminum-tolerant varieties and crops, yield will still often be lower than if they were grown on soils with a pH Acidification in the Palouse closer to neutral. Acidification is an increasing concern in the Palouse as Managing Low pH production of high yielding wheat crops has required substantial nitrogen fertilizer inputs. Nitrogen fertilizer use in Amending soils with agricultural lime is the most common and the entire US has been rapidly rising since the 1960s, as shown effective long- and short-term strategy to correct soil acidity. in Figure 7. In the early 1980s, University of Idaho researcher, Other short-term strategies that can mitigate the impact of low Dr. Robert Mahler expressed concern after a survey of soil pH on crop yield include planting aluminum-tolerant northern Idaho and eastern Washington soils showed a trend varieties of wheat or aluminum-tolerant crops such as triticale. with over 65% acidification, to below pH 6, in several counties between 1972 and 1980 (Mahler et al, 1985). Today, farmers, researchers and extension personnel are finding many sites in the PNW with a surface soil pH below 5.0. In extreme cases, pH of the fertilizer zone has been measured at lower than 4.0. A growing trend of extremely low pH values in Palouse soils represents a serious threat to land quality and represents the degradation of a significant capital investment for farms throughout the region.